1. Field of the invention
The present invention relates to a method of fixing to a recording medium a visible image formed with a toner, as in image forming processes such as electrophotography, electrostatic printing, and magnetic recording.
2. Related Background Art
As a method of fixing a visible image of toner onto a recording medium, a heat-roll fixing system is widely used, in which a recording medium retaining thereon a toner visible image having not been fixed is heated while it is held and carried between a heat roller maintained at a given temperature and a pressure roller having an elastic layer and coming into pressure contact with the heat roller.
As another method, a belt fixing system is known, as disclosed in U.S. Pat. No. 3,578,797.
The above conventional heat-roll fixing, however, has the following disadvantages:
(1) A long waiting time is required until the heat roller reaches the given temperature. PA1 (2) The heat roller must be maintained at an optimum temperature in order to prevent poor fixing caused by the variations of the heat-roller temperature that may occur when the recording medium is passed or because of other external factors, and also to prevent the offset phenomenon of toner on the heat roller. This makes it necessary to make large the heat capacity of the heat roller or a heater element, which requires a large electric power. PA1 (3) When the recording medium is passed over the heat roller and delivered out, the recording medium and the toner on the recording medium are slowly cooled, resulting in a state of high adhesion of the toner. Thus, conjointly with the curvature of the roller also, there may often occur offset, or paper jam caused by the rolling-up of the recording medium. PA1 (4) A protective member must be provided in order to prevent direct touch to the high-temperature heat roller.
On the other hand, the above problems (1) and (2) of the heat-roll fixing are not fundamentally solved also in the belt fixing system disclosed in U.S. Pat. No. 3,578,797.
Japanese Patent Application No. 147884/1987 (corresponding to European Publication No. 0295,901), as having already been proposed by the present applicant, proposes an image forming apparatus with a shorter waiting period and a low power consumption, comprising a fixing unit in which a toner visible image is heated through a movable heat-resistant sheet by means of a heating element having a low heat capacity, pulsewise generating heat by applying electric power, and thus fixed to a recording medium. Japanese Patent Application No. 63-12069 (corresponding to European Publication No. 0295,901), as also having already been proposed by the present applicant, proposes a fixing unit for heat-fixing a toner visible image on a recording medium through a heat-resistant sheet, wherein said heat-resistant sheet comprises a heat-resistant layer and a release layer or a low-resistant layer, thereby effectively preventing the offset phenomenon.
In addition to the fixing unit mentioned above, properties of the toner are greatly concerned in realizing a fixing method that requires only a short waiting period and a low power consumption while achieving the excellent performance of fixing a toner visible image to a recording medium and the prevention of the offset phenomenon.